Information storage systems, particularly computer memory systems, typically store data magnetically or optically onto several types of media, such as rotating disks. Data stored on such disks, whether magnetic or optical, is contained within a series of tracks. Once formed on a disk, such tracks are typically spiral or concentric shaped and can number up to several thousand tracks on each side of a disk, depending on the diameter of the disk utilized and whether the information is recorded magnetically or optically. The tracks on a disk can be viewed as roughly analogous to grooves on a phonograph record.
In magnetic recording and magneto-optical recording, information is stored on a subject media by orienting the magnetic field of the media at given points along given tracks. In order to access or read data stored on a disk, a so-called head or transducer is moved along a generally radial path across the surface of the disk as the disk is spinning. The generally radial movement will either follow a straight line path or an arcuate path, depending on whether a linear or rotary actuator is used for positioning the head.
In magneto-optic storage, similar to magnetic recording, information is encoded and stored in a sequence of bits defined by magnetic domains oriented normal to the media surface in either of two possible orientations, either north-pole-up or north-pole-down, for example. A blank disc, i.e. an erased track, has all of its magnetic poles oriented in one direction. On magneto-optic media the magnetic force required to flip one magnetic domain from, for example, north-pole-down to north-pole-up, i.e., the coercive force, varies greatly with temperature. At room temperature, the coercive force necessary to flip the magnetic media is so high that an ordinary magnet is too weak. At approximately 150'C, the coercive force required to flip a domain decreases substantially (200-450 Oe) and a bit can be recorded using ordinary magnets including electromagnetics.
During a recording operation in a magneto-optic system, a focused laser beam is used to heat selected spots on the media near a relatively large electro-magnet. In this way, a point on the media can be heated, lowering the coercive force required to write a bit of information and the magnet, depending on the direction of flux generated by such magnet, can record the desired bit. Once the laser beam is turned off, the previously heated point on the media cools "freezing" the oriented media in the desired orientation. To erase information so recorded, the process need only be reversed, that is the point on the media will be heated by the laser beam and the direction of flux generated by the magnet will be such to re-orient media based north magnetic poles in a single orientation.
Referring to FIG. 1, a more graphical interpretation of the above magneto-optical system will be described. A magneto-optical disk 10 is depicted with a small portion of disk 10 enlarged and presented in perspective. Those skilled in the art will recognize that a transparent substrate layer, normally present on disk 10, has not been shown. The enlarged portion 12 is shown to have a magneto-optic layer 14 which overlays an encapsulation layer 16. Had the transparent layer been depicted, it would have overlayed magneto-optic layer 14. Disk portion 12 is shown to have a series of concentric tracks 18a, 18b, 18c and 18d. The tracks are depicted as raised having a valley or groove 20 therebetween.
Reading information recorded on a magneto-optic disc is achieved solely through electro-optical means. A lower power light beam from laser diode 22 is collimated by lens 24 passed through a polarization or "leaky" beam splitter 26a and focused by lens 28 onto track 18b. Depending on the type of media utilized, the laser light beam may be either reflected from magneto-optic layer 14 or transmitted through the layer and, respectively, is read from either above or below the media. Because of phenomena known as the Kerr magneto-optical effect and the Faraday effect, light reflected from the media (Kerr) or transmitted through the media (Faraday) will have a slightly different polarization state than the incident light focused onto the media. The change in polarization state will, typically, comprise rotation of the plane of polarization of linearly polarized light and introduction of ellipticity depending on the orientation of the media at that point.
As shown in FIG. 1, light reflected from track 18b is collimated by lens 28 and reflected by polarization beam splitter 26a to amplitude beam splitter 26b. Beam splitter 26b divides the polarized light into first and second beams for differential detection purposes. The first beam is focused by positive servo lens 29a onto the surface of detector 30a. The second beam is focused by analyser 29b onto the surface of detector 30b. See generally Freese, Robert P., "Optical disks become erasable", IEEE Spectrum, February, 1988, pages 41-45. In differential detection the electrical signals generated by detectors 29a and 29b are subtracted.
As will be appreciated from the above, when reading or writing information onto a magneto-optical disk or any optical disk, it will be necessary to maintain the positioning of the light beam focused by lens 28 on track 18b as disk 10 rotates. Such an operation is known as track following. Track following requires the generation of a radiel position error signal. It will also be appreciated from the above that because relatively small magnetic domains will be recorded, read and erased, it is important to maintain a focused beam of light on the desired track. Maintaining the focus of the light beam requires the generation of a focus axial error signal. Each of these signals, the position error signal and the focus error signal, are calculated based on signals generated by detector 30.
Light reflected from grooved disk 10 and directed onto detector 30 will form a sheared interferogram. When light is focused on a spot on grooved media, such as that used in optical and magneto-optical disks, the reflected light contains a series of orders of reflection each having an axis deviated from the central axis. These reflection orders overlap producing the sheared interferogram. A sheared interferogram is directed onto a detector, such as detector 30. When sampled properly, the detected sheared interferogram can be used to generated the radial position error signal and the focus error signal. The ability to calculate these error signals is based upon the properties of the sheared interferogram in relation to focus and radial position.
The sheared interferogram generated by the magneto-optical system shown in FIG. 1, is depicted in FIG. 2 on detector 30. The sheared interferogram is shown to include the zero order reflection 32 and a portion of the first order reflections 34 and 36. As shown, the first order reflections just "touch" in the center of the zero order reflection 32. In relation to the position of the focused beam of light on track 18b, the configuration of the sheared interferogram will remain basically the same, i.e., the first order reflections will just "touch" in the center of the zero order reflection. However, the light and dark areas associated with the sheared interferogram will change. Since the focused light in FIG. 1 is shown to be "on-track" with respect to track 18b, the sheared interferogram of FIG. 2 is depicted as having equally dark regions in the areas of overlap between the zero order and first order reflections. As the beam of light is moved radially across disk 10, the shading in the sheared interferogram shown in FIG. 2 will change. Referring to FIG. 3, there is shown the changes in the shading of the sheared interferogram as the beam of light focused by lens 28 onto disk 10 is moved radially.
FIG. 3a shows a sheared interferogram which indicates that the light focused by lens 28 has moved radially inwardly in relation to disk 10 such that it is in the region of edge 38, shown in FIG. 1. In such a location, it will be seen that overlap portion 34 is dark shaded while overlap 36 has no shading. Referring now to FIG. 3b, the beam of light focused by lens 28 has moved still further radially inwardly such that it is now positioned in groove 20. As shown in FIG. 3b, overlap portions 34 and 36 are equally shaded, indicating that the beam of light is located in a central location. FIG. 3c depicts a sheared interferogram which is representative of the beam of light focused by lens 28 being located in the region of edge 40. When the focused light is in such a position, the sheared interferogram shows overlap portion 34 as having no shading and overlap portion 36 as having shading. Knowing the properties of the sheared interferogram in relation to the positioning of the focused beam of light, one can utilize detector 30 to generate a radial position error signal.
Referring back to FIG. 2, it will be seen that detector 30 is actually a combination of four detectors, each having photosensitive surfaces. Each of the four detectors will generate a signal indicative of the intensity of the light on its surface. Each of the four detectors has been designated A, B, C and D. The radial position error system is generated by adding and subtracting signals generated by these detectors. In accordance with the detectors shown in FIG. 2, the position error signal (PE) can be determined according to the following formula: EQU PE=B-C
In order to determine focus, detector 30 in effect senses the diameter of the illuminated spot, including the sheared interferogram. Accordingly, focus is determined in effect by sensing the size of the spot formed by the intersection of the detector with the converging beam. As shown in FIG. 2, focus is determined by amplifier 42 in accordance with the following formula: EQU FE=(A+D)-(B+C)
where FE is the focus error signal.
It will be noted that detector 30 is known as an "I" type detector. In addition to the I type detector, quadrant type detectors have also been proposed for use in determining position error signals and focus error signals. See for example, U.S. Pat. Nos. 4,773,053 - Gottfried, 4,797,868 - Ando and 4,779,250 Kogure, et al.; and Lee, Wai-Hon, "Optical Technology For Compact Disk Pickups", Lasers and Optronics, pp. 85-87 (September 1987).
Although the above described devices for generating position error and focus error signals are generally adequate, they do exhibit problems where fine focusing is required. It is apparent from the above that utilizing a focused spot of light has an advantage high-density of information storage. It has been estimated that the theoretical upper limit of the storage capacity of magneto-optical systems can be as high as 300 megabits per square inch of media. In practical terms, on a 5.25 inch floppy disk, yields of approximately 400 to 800 megabits can be expected. The problem associated with utilizing a strongly converging cone of light is the inherent shallow in-focus region. Typically, the focused region for the cone of light utilized in optical information storage systems is on the order of one micrometer. The above described devices, although generally acceptable, cannot maintain a precisely focused spot within this one micrometer focused region, but suffer from inadequate sensitivity in some cases. Consequently, a need still remains for a detector which is capable of not only providing position error information but also of providing focus error information which can be used to coarsely focus the light beam and thereafter fine focus the light beam.